The present invention relates to a process for making high strength, low shrinkage polyamide yarn and yarn made thereby and more particularly relates to method for making a high strength, low shrinkage polyamide yarn in a coupled, high-speed spin/draw process which is useful for high volume production of industrial polyamide yarns.
High strength polyamide yarns are useful for a wide variety of industrial applications including use as reinforcement in tires, conveyor belts, hoses, and other reinforced rubber goods, use in plastic-coated fabrics, and in ropes, cordage, webbing, and woven fabrics. For reasons including cost reduction, energy savings, fabric yield, and end use product safety, converters of polyamide yarns to such uses desire the yarn to have a combination of properties including high tenacity and low shrinkage upon heating. Particularly desirable properties are a tenacity of at least about 9.5 g/d and a shrinkage of less than about 4.0%. This combination of properties is difficult to produce, particularly in a high speed, coupled spin/draw process for high volume commercial production.
Known coupled spin/draw processes for making multifilament polyamide yarns of high tenacity draw the spun filaments in at least two stages. In some known processes, the yarn is first drawn without heating often using snubbing pins in a first drawing stage. The yarn is then heated in a second draw stage to enable drawing to greater than 5.0X draw ratio required for high tensile properties.
In processes of this type, the shrinkage will usually be quite high (over 10%) unless process steps are taken to decrease the shrinkage to a desired extent. The shrinkage can be significantly reduced by heating the yarn at high temperature at constant length and subsequently allowing it to retract/relax at elevated temperatures. In the coupled spin/draw/anneal process described in U.S. Pat. No. 3,311,691, the yarn is annealed by the hot second (and final) stage draw rolls and allowed to relax by operating the tension letdown rolls at a slower speed than the second stage rolls. The amount of retraction/relaxation of the yarn obtained in a process of this type can be described as percent (%) letdown which is defined for the purposes of this application as: ##EQU1## There is an inverse relationship between % letdown of the spinning process and the resulting yarn shrinkage, i.e., a high % letdown results in a low shrinkage.
In the process of U.S. Pat. No. 3,311,691, it is difficult to string-up and operate a high speed coupled spin/draw process at a high % letdown. String-up and commercial continuity of operation cannot be readily accomplished at greater than approximately 8% letdown since the wraps of yarn on the tension letdown rolls are at too low tension. The insufficient tension can either cause the yarns to become entangled and form a wrap band or one of more filaments may wrap on one of the rolls. In either case, the process must be shut down. While the tension letdown rolls ultimately become heated from the heat carried by the advancing yarn (temperatures on the order of 110.degree. C.) which enables a higher % letdown, it is generally not possible to employ a % letdown of greater than about 9%. Accordingly, using the process of U.S. Pat. No. 3,311,691, shrinkages usually cannot be decreased to below about 5.5% while maintaining tenacities above about 9.5 g/d.